Fuel element fabrication method



May 31, 1960 J- N. HIX ET AL FUEL ELEMENT FABRICATION METHOD OriginalFiled Aug. '7, 1956 INVENTORS. George E. Co BY John Jose oley E.Cunningham ph N. Hix 4 M 4am ATTORNEY FUEL ELEMENT FABRICATION METHODJoseph N. Hix, Kingston, John E. Cunningham, Oak

Ridge, and George E. Cooley, McMinnville, Tenn., assignors to the UnitedStates of America as represented by the United States Atomic EnergyCommission 2 Claims. (Cl. 204--154.2)

This invention relates to an improved method of assembling andfabricating a multiplate neutronic reactor fuel element of the generaltype disclosed in The Reactor Handbook, vol 2, pp. 243 and 244, and thisapplication is a continuation of our prior application SIN. 602,683,filed August7, 1956, now abandoned.

As there disclosed, such a fuel element comprises a plurality of spacedparallel fuel plates of a bowed configuration supported by and between apair of transverse aluminum side plates. The fuel plates each comprisean inner layer of uranium-aluminum alloy which is surrounded on allsides by a protective outer layer of aluminum. The fuel plates fit intoaccommodating parallel grooves cut in the inner opposing faces of theside plates. Such fuel elements have come to assume an important placein the neutronic reactor art, and are presently employed in manyoperating reactors, including the materials testing reactor described inreport T.I.D.

7001, Materials Testing Reactor Handbook, edited by J. H. Buck and C. F.Leyse.

The prior art method of assembling and fabricating these fuel elementsinvolved a conventional brazing technique wherein individual strips ofbrazing alloy were laid on opposite sides of each fuel plate adjacentthe associated grooves of the supporting side plates preparatory to thebrazing heat treatment, the entire assembly being temporarily supportedat this time in a jig, and the individual fuel plates being supported bythe grooves in the side plates. The grooves had to be sufficiently deepto provide temporary support for the fuel plates during the initialassembly, taking into account the slight unavoidable variations in widthof the individual fuel elements. At the same time, the thickness of theside plate beyond the groove had to be sufiicient to provide structuralrigidity and strength to the finished fuel element. Thus, the requiredtotal thickness of the side plate was determined, in part, by the depthof the groove necessary to provide temporary support for the fuelelements during initial assembly.

In accordance with the principles of the present invention, the brazingalloy is preplaced on one surface of the aluminum side plates in theform of a cladding or layer of uniform thickness. The grooves are thencut in the side plates through the alloy layer and into the basealuminum. Since a portion of the necessary groove depth is thus suppliedby the brazing alloy, the brazing alloy, itself, assists in thetemporary support of the fuel plates during the preliminary assembly.This permits utilizing thinner aluminum side plates, which is, ofcourse, highly desirable in reactors of the materials testing reactortype, since a lower aluminum-to-water ratio for the reactor is therebyobtained. Also, since the depth of the groove in the aluminum baseportion of the side plate is smaller (approximately one-half of thegroove depth previously required), the brazing flux is more efiiciently5 flushed out by the brazing alloy during the subsequent brazing heattreatment. Since the brazing flux necessarily contains elements having ahigh neutron absorption 2,938,846 Patented May 31, 1960 2 cross-section,such as lithium, this more eflicient flushing out of the brazing flux isof special advantage. The smaller depth of groove in the aluminum baseportion'of the side plate also results in a lesser amount of the brazingalloy, itself, being required to fill the groove and being incorporatedin the finished fuel element. This is of considerable advantage sincethe brazing alloy contains considerable silicon which interferes withthe chemical processing of the fuel element which is ultimatelynecessary to recover unused fuel. In addition to the above advantages,preplacing of the brazing alloy is much more convenient and eflicientthan the time consuming process of manually placing individual strips ofbrazing alloy at their proper positions, and it is much more adapted toa controlled high quality production type operation producing uniformfuel elements containing minimum contaminants and extraneous material.

An object of the invention, therefore, is to provide an improved methodof fabricating multiplate fuel elements of the materials testing reactortype which permits-the use of thinner aluminum side plates.

Another object of the invention is to provide an improved method offabricating multiplate fuel elements of the materials testing reactortype which permits the use of shallower grooves in the aluminum sideplates.

Another object of the invention is to provide an im proved method offabricating multiplate fuel elements'of the materials testing reactortype which results in the final product containing less brazing fluxconstituents and less brazing alloy.

Still another object of the invention is to provide an improved methodof fabricating multiplate fuel elements .of the materials testingreactor type which is more effcient and more adapted to a productiontype operation providing a uniform quality controlled finished product.

These and other advantages will become more apparent from the followingdescription, when-taken in connection with the accompanying drawing,wherein:

Figure l is an end view of a fuel element positioned in a drying'jigduring initial assembly;

Figure 2 is a fragmentary cross-sectional view showing the spatialrelationship between a fuel plate and associated side plate grooveduring initialassembly;

Figure 3 is a fragmentary cross-sectional view showing the spatialrelationship between a fuel plate and associated side plate grooveduring a subsequent stage in the fabrication process; I v

Figure 4 is an end view of a fuel element positioned in a refractorybrazing jig; and

Figure 5 is a fragmentary cross-sectional view showing the spatialrelationship between a fuel plate and associated side plate groove afterbrazing. l 1

Referring now to the drawings, reference numeral 1 designates the fuelelement generally, reference numeral 2 designates the plurality of fuelplates, and reference numeral 3 designates the supporting side plates.As shown in Figs. 3-5, each fuel plate 2 consists of an innerfissionable material containinglayer 4,'for example, a 10-20% enriched'uranium8090% aluminum alloy, surrounded on all sides by a protectiveouter cladding 5 of aluminum. The fabrication of the fuel platesZ,themselves, forms no part of the present invention, and may beaccomplished by methods described in MTR-.T.ype Fuel Elements, J. E.Cunningham and: E. J. Boyle; a paper released in connection with the,August 1955 Geneva Conference and designated as A/ Conf./ .8/ P/ 953.

The side plates 3 of the final product are formed sole- 1y of aluminum,as shown in Figure 5, this Figure 5 also showing the spatialrelationship, in the final product, between the fuel plate 2, theassociated groove 6 in the side plate, and the brazing alloy 7.

In accordance with the principles of the present inveu tion, the brazingalloy, which, for example, may constitute a 12% Si-88% Al alloy, ispreplaced upon one surface of the side plate 3 in the form of a claddingS of uniform thickness, as most clearly shown in Figures 2 and 3.Braze-clad aluminum'sheet material may be fabricated by rolling togethera base sheet of aluminum and a sheet of the braze alloy at 525550 C. Thebrazeclad side plates 3, 8 may then be cut to sizefrom this sheetmaterial. Typically, the thickness of this composite side plate, withcladding, is about 125 mils, and the thickness of the brazing alloycladding, itself, is about -19 mils. a After the braze-clad side plates3, 8 is properly dimensioned, the parallel grooves 6 are cut by gangmilling, the grooves extending entirely through the brazing alloycladding 8 and part way into the aluminum side plate ,3 proper, as shownin Figures 2 and 3. The braze-clad side plates 3, 8 are then coated witha slurry of a commercial brazing flux and alcohol. The brazing fluxdisclosed in U.S. Patent No. 2,481,053, Welding or Brazing FluxComposition, Rene Wassermann, September 6, 1949, may, for example, beused, this flux comprising aeutectic mixture of chlorides and/orfluorides of sodium, lithium, potassium, and cadmium.

The fuel element 1 is then initially assembled in the metal drying jig10 (see Figure l), the distance between the upstanding sides 11 of whichis adjustable by means of bolts 12. The drying jig 10 is first opened upslightly, as illustrated in Figure l, and the two braze-clad 7 sideplates 3, 8 are placed against the sides 11 of the jig. In the openposition of the ji-g 10, the fuel plates 2 can be inserted endwise intothe appropriate grooves and slid into place. At this time, the fuelplates 2 are supported solely by the grooves 6 in the braze-clad sideplates 3, 8, and at least one side of at least some of the platesreceives its sole support from the brazing alloy cladding 8. Figure 2illustrates how one typical fuel plate 2 may be spatially related to anassociated groove 6 at this stage in the fabrication process,theillustrated fuel element being supported at one edge solely by thebrazing alloy-cladding 8.

After all fuel plates 2 have been inserted and properly aligned, thedrying jig 10 is tightened. The fuel ele ment illustrated in Figure 12is thereby caused to assume that spatial relationship to the groovewhich is shown in Figure 3.

The drying jig 10 with its assembled fuel element 1 is then subjected toa three to four hour heat treatment as about 150 C. After this heattreatment, the fuel element '1, due to the tackiness of the brazingflux, has sufficient strength to support itself and to be manuallyhandied without falling'apart.

The fuel element is then manually transferred to the selfaligningbrazing jig, as shown in Figure 4. The brazing ,jig consists simply of apair of upper and lower generally U-shaped members 13 and a pair ofcooperating side panels 14, all these pieces being separable. Thebrazing jig forms an elongated rectangular enclosure of a size justsufiicient to accommodate thefuel element 1. The brazing jig is formedof a suitable refractory mate rial.

The brazing jig and its included fuel element is then preheated forabout an hour at 500 C., and then subjected to a brazing heat treatmentat about 610" C. for twenty-five minutes in a forced-circulationfurnace. During the brazing process, the brazing alloy forming thecladding 8 becomes molten and fiows down to the next lower fuel plate 2and into and around the associated groove 6, capillary action causing itto fill the groove vol. 9, August 1955, pages 203 -207.

2,938,846 I V K completely. After cooling to room temperature, the fuelelement 1 is then washed to remove the water soluble brazing flux,cleaned in.an acid bath, rinsed in hot water, and dried. Figure 5illustrates the spatial relationship of the fuel plate 2 and groove 6 inthe finished fuel element, the brazing alloy7 completely filling thegroove 6 and extending outwardly a little along the surface of the fuelplate.

Since many changes could be made in the above fabrication method andmany apparently widely different embodiments of this invention could bemade without departing from the principles thereof, it is intended thatall matter contained in the above description, or shown in theaccompanying drawing, shall be interpreted as illustrative and not in alimiting sense.

What we claim is: v I

l. in a method of fabricating a reactor fuel element of the type whichcomprises a plurality of spaced parallel aluminum clad fissionablematerialcontaining fuel eleemnts supported by, and between, a pair ofparallel aluminum side plates, the steps of hot rolling on one side of asheet of aluminum a sheet of brazing alloy having a melting point lowerthan that of the sheet of aluminum, forming the side plates from theresulting braze-clad aluminum sheet, cutting a plurality of grooves inthe alloy side of the side plates through the alloy and extending intothe aluminum base plate, positioning the two side plates within a dryingjig vertically on edge a fixed distance apart with their groovedsurfaces facing one another and the grooves running horizontally,assembling the fuelelement by sliding the fuel plates endwise into theassociated grooves, the distance between said side plates during theassembly step being such that at least some of the fuel plates aresupported on at least one side solely by the brazing alloy, moving saidside plates toward one another such that all of the fuel plates aresupported solely by the base aluminum of the side plates, subjecting theassembly to a drying treatment to remove liquids, then removing theassembly from the drying jig and inserting it in a brazing jig,andsubsequently subjecting the as sembled fuel element to a brazing heattreatment to raise the temperature of the brazing alloy to the pointwhere it forms a bond between the fuel plates and the side plates in andaround the grooves.

2. In a method of fabricating a reactor fuel element of the type whichcomprises a plurality of spaced parallel aluminum clad fissionablematerial containing fuel elements supported by, and between, a pair ofparallel aluminum sideplates, the steps of hot rollinga sheet of an 88%aluminum-12% silicon brazing alloy on one side.

of a sheet of aluminum, forming the side plates from the resultingbraze-clad aluminum sheet, cutting'a plurality of grooves in the alloyside of the side plates through'the alloy and extending into thealuminumbase'plate, positioning the side plates in a drying jig, thenpositioning the fuel plates between the pair of sides plates withopposing edges of the fuel plates engaging opposing groove'sof the sideplates, subjecting the'assem'bly to a drying treatment in the C. range,then inserting the assembly in a ceramic brazing jig, then preheatingthe assembly at a temperature within the 500 C. region, and subjectingthe fuel element to a brazing heat treatment to raise the temperature tothe point where a bond is formed between the fuel plates and sideplates. i

References Cited in the file of this patent International Conf. onPeaceful Uses of Atomic Energy,

1. IN THE METHOD OF FABRICATING A REACTOR FUEL ELEMENT OF THE TYPE WHICHCOMPRISES A PLURALITY OF SPACED PARALLEL ALUMINUM CLAD FISSIONABLEMATERIAL CONTAINING FUEL ELEEMNTS SUPPORTED BY, AND BETWEEN, A PAIR OFPARALLEL ALUMINUM SIDE PLATES, THE STEPS OF HOT ROLLING ON ONE SIDE OF ASHEET OF ALUMINUM A SHEET OF BRAZING ALLOY HAVING A MELTING POINT LOWERTHAN THAT OF THE SHEET OF ALUMINUM, FORMING THE SIDE PLATES FROM THERESULTING BRAZE-CLAD ALUMINUM SHEET, CUTTING A PLURALITY OF GROOVES INTHE ALLOY SIDE OF THE SIDE PLATES THROUGH THE ALLOY AND EXTENDING INTOTHE ALUMINUM BASE PLATE, POSITIONING THE TWO SIDE PLATES WITHIN A DRYINGJIG VERTICALLY ON EDGE A FIXED DISTANCE APART WITH THEIR GROOVEDSURFACES FACING ONE ANOTHER AND THE GROOVES RUNNING HORIZONTALLY,ASSEMBLING THE FUEL ELEMENT BY SLIDING THE FUEL PLATES ENDWISE INTO THEASSOCIATED GROOVES, THE DISTANCE BETWEEN SAID SIDE PLATES DURING THEASSEMBLY STEP BEING SUCH THAT AT LEAST SOME OF THE FUEL PLATES ARESUPPORTED ON AT LEAST ONE SIDE SOLELY BY THE